dsh-triangle: Triangle Computation

`dsh-triangle` is a pure Node.js triangle calculation plugin designed specifically for DeepSeek Harness (DSH) agent applications. It does not rely on network requests and injects calculation capabilities into sessions through registered tool interfaces. The plugin depends on `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools`, and can be invoked directly after installation. It is suitable for specialized logic-processing scenarios such as mathematical calculations.